Liquefied natural gas (“LNG”) has allowed the supply of natural gas from locations with an abundant supply of natural gas to distant locations with a strong demand for natural gas. The conventional LNG cycle includes: (a) initial treatment of the natural gas resource to remove contaminants such as water, sulfur compounds, and carbon dioxide; (b) separation of some heavier hydrocarbon gases, such as propane, butane, and pentane, from the natural gas, where the separation can occur by a variety of possible methods including self-refrigeration, external refrigeration, or lean oil, etc.; (c) refrigeration of the natural gas to form liquefied natural gas at near atmospheric pressure and about −160° C.; (d) transport of the LNG product in ships or tankers to a market location; and (e) re-pressurization and re-gasification of the LNG at a regasification plant to a pressure at which natural gas may be distributed to natural gas customers. Step (c) of the conventional LNG cycle typically uses external refrigeration which requires the use of large refrigeration compressors often powered by large gas turbine drivers that can produce greenhouse gas emissions. Thus, a large capital investment is typically needed to put in place the extensive infrastructure needed for the liquefaction plant. Step (e) of the LNG cycle generally includes re-pressurizing the LNG to the required pressure using cryogenic pumps and then re-gasifying the LNG to pressurized natural gas by exchanging heat through an intermediate fluid, such as seawater, or by combusting a portion of the natural gas to vaporize the LNG.
A cold refrigerant produced at a different location, such as liquefied nitrogen gas (“LIN”), can be used to liquefy natural gas. For example, U.S. Pat. No. 3,400,547 describes shipping liquid nitrogen or liquid air from a market place to a field site where it is used to liquefy natural gas. The LNG is shipped back to the market site in the tanks of the same cryogenic carrier used to transport the liquefied nitrogen or air to the field site. Regasification of the LNG is carried out at the market site, where the excess cold from the re-gasification process is used to liquefy nitrogen or air for shipping to the field site.
However, since the natural gas from the regasification of LNG must be at a higher pressures (e.g., greater than 800 psi) for introduction into the gas sales pipeline, the total energy needed for both the production of LIN and the re-pressurization of natural gas can be significantly greater than the energy needed to produce LNG using conventional processes. Therefore, there is a need to develop more energy efficient methods to produce LIN and high pressure natural gas from the regasification of LNG.
Furthermore, the process of U.S. Pat. No. 3,400,547 requires the integration of the complete LNG value chain. That is, there must be integration of the production of LNG using LIN as the cold refrigerant, the shipping of LIN to the natural gas resource location, the shipping of LNG to regasification locations, and the production of LIN using the available exergy from the regasification of LNG. This value chain is further described in U.S. Patent Application Publication Nos. 2010/0319361 and 2010/0251763.
The production of LNG at the gas resource site using LIN as the sole refrigerant may require a LIN to LNG ratio of greater than 1:1. For this reason, the production of LIN at the regasification site favors a greater than 1:1 LIN to LNG ratio in order to ensure that only the LNG produced using the LIN is then required to liquefy the needed amount of nitrogen. The matching of the LIN to LNG ratio at both the LNG plant and the regasification plant allows for an easier integration of the LNG value chain since LNG from additional production sources is not needed.
GB Patent Application Publication No. 2,333,148 describes a process where the vaporization of LNG is used to produce LIN, where the LIN to LNG ratio that is used is greater than 1.2:1. In GB Publication No. 2,333,148 the LNG is vaporized close to atmospheric pressure. Therefore, since the standardized pressure at which LNG must be when entering the gas sales pipeline is greater than 800 psi, a significant amount of energy is required to compress the natural gas to pipeline pressure. As such, there is a need for a method which allows pumping the LNG to higher pressures prior to vaporization in order to minimize the required amount of natural gas compression.
GB Patent 1,376,678 and U.S. Pat. Nos. 5,139,547 and 5,141,543 describe methods where LNG is first pressurized to the pipeline transport pressure prior to vaporization of the LNG. In these disclosures, the vaporizing LNG is used to condense the nitrogen gas and is used as the interstage coolant for the multistage compression of the nitrogen gas to a pressure of at least 350 psi. The interstage cooling of the nitrogen gas using the vaporizing and warming of the natural gas allows for cold compression of the nitrogen gas which significantly reduces its energy of compression. However, in these disclosures a LIN to LNG ratio of less than 0.5:1 is used to produce the LIN and high pressure natural gas. This low LIN to LNG ratio does not allow for point-to-point integration of the regasification plant with the LNG plant, since a LIN to LNG ratio of at least 1:1 is typically required to produce LNG using LIN as the sole refrigerant.
U.S. Patent Application Publication No. 2010/0319361 describes a method where LNG from multiple production sources are used to produce the LIN needed for LNG production at one production site. However, this multi-source LNG value chain arrangement significantly complicates the LNG value chain.
Therefore, there remains a need to develop an energy efficient method for producing LIN and high pressure natural gas from the regasification of LNG. There is further a need for an integrated method that is able to utilize a LIN to LNG ratio that is greater than 1:1, or more preferably greater than 1.2:1.
Other background references include GB Patent No. 1596330, GB Patent No. 2172388, U.S. Pat. Nos. 3,878,689, 5,950,453, 7,143,606, and PCT Publication No. WO 2014/078092.